1. Field of the Invention
The invention relates to a method of optically writing and subsequent reading and/or erasing information in a recording plane of an optical record carrier having at least two recording planes and a guide plane. A guide beam focused to a spot (guide focus) in the guide plane is used during writing, and at least one write beam focused to a writing spot (write focus) in the recording planes, the guide focus and the write focus being formed by one objective system and the guide focus being held in the guide plane by means of a focus error signal generated by the guide beam.
A multi-layer record carrier comprises a stack of information layers separated from one another by spacer layers, in which each information layer may comprise information. The large information storage capacity of such a record carrier increases its convenience of use compared to single-layer record carriers and reduces the price of the medium per unit of information. Each information layer can be scanned independently of the other information layers by means of a radiation beam. Dependent on the type of record carrier, information can be written into an information layer during scanning and/or information already written can be read or erased. The information layers in a stack can be scanned by means of a radiation beam which is incident from one side on the record carrier. For scanning the separate information layers, the height, or axial position, of the scanning spot formed by the radiation beam is varied. The information contents of the record carrier may be further increased by implementing the record carrier as a two-sided record carrier. Then a stack of information layers is present at both sides of the record carrier, and each stack can be scanned from a different side of the record carrier. A stack of information layers may be provided on a substrate which should be transparent if the stack is scanned through the substrate.
2. Description of the Prior Art
A method of the type mentioned in the opening paragraph is described in Japanese Patent Application 63-234418. In accordance with this method an objective system converges a guide beam to a guide focus on a guide plane in a record carrier. A focus servosystem controls the objective system in such a way that the guide focus remains in the guide plane in spite of possible excursions of the record carrier. A read or write beam, or generally a scanning beam, is focused by the objective system on a recording plane to be written or read, which plane is parallel to the guide plane. For this purpose the read/write focus of a read/write beam formed by the objective system must be displaceable with respect to the guide focus in the longitudinal direction, i.e. in the direction of the optical axis. Starting from a reference position of the scanning focus, which reference position is equal to the desired position of the guide focus, this is realised by displacing the radiation source supplying the scanning beam along the optical axis over discrete distances which match the distances between the recording planes.
To be able to use the known method with a so-called passive longitudinal adjustment of the scanning focus, the different recording planes of the record carrier must be very accurately parallel to the guide plane within the focus depth of the objective system, because otherwise the scanning focus is not always located in a recording plane to be scanned. A multilayer record carrier having such a high degree of parallelism of the layers is difficult to manufacture and is consequently expensive. Moreover, during writing, the scanning focus should accurately follow a given track in a recording plane to be scanned, while during reading the scanning focus should accurately follow the written information tracks. Japanese Patent Application 63-234418 does not reveal how this so-called transversal positioning of the scanning focus must be realised.
An information storage system of the type described in the opening paragraph is known from European Patent Application no. 0 517 491. in which a device is described for reading information lovers in a multi-layer record carrier. The device is provided with an adjustable spherical aberration compensator for compensating the spherical aberration incurred by the radiation beam of the device when it passes through the material of the record carrier. Since the information layers are located at different heights in the record carrier. the device employs a specific setting of the compensator for each information layer. A drawback of this known information storage system is that there should be a separate compensation for each information layer. A compensator which can realise this is complicated and relatively expensive. The relatively low cost of the record carrier per unit of information is thus counteracted by a relatively expensive scanning device.
An object of the invention is to provide a method and apparatus of the type described in the opening paragraph in which a record carrier which can easily be manufactured can be used and in which the transversal positioning problem is also solved.
Another object of the invention to provide an information storage system having a relatively low price and at the same time a high information density.
The method
In accordance with a first aspect of the invention the method is characterized in that during writing the transversal position of the write focus in a recording plane is coupled to the transversal position of the guide focus, the latter position being controlled by a tracking error signal obtained from the cooperation between the guide, beam and the guide plane, and in that during reading and/or erasing:
a) a read focus formed by a read beam is held in the scanned recording plane by means of a focus error signal obtained from the cooperation between the read beam and the scanned recording plane,
b) the focusing means introduce such a fixed, stack-associated spherical aberration in the radiation beam that this aberration compensates the spherical aberration incurred by the radiation beam when it is focused at approximately half the height of the stack of information layers, and
c) the transversal position of the read focus is controlled by a tracking error signal obtained from the cooperation between the read beam and the scanned recording plane.
The invention is based on the recognition that the transversal position of the write focus can be controlled by coupling this position to that of the guide focus by means of guide information in only one plane of the record carrier when writing all recording layers, and that the read focus can be controlled independently of the guide focus when reading written recording layers. Further, a stack of information layers can be scanned by a satisfactorily corrected scanning spot, while using a single, constant and suitably chosen spherical aberration compensation. Since the spherical aberration is not compensated anymore for each information layer individually as in the known system, the focusing means can be made simpler, reducing the cost of the scanning device. The spherical aberration incurred by a focused radiation beam as a function of the thickness of the material through which the beam passes appears to be sufficiently small for a reasonably large range of thicknesses, which range is located symmetrically around the thickness for which the radiation beam is well compensated. By compensating the radiation beam in such a way that the scanning spot is substantially free from spherical aberration at approximately half the height of the stack, it is possible to scan information layers located within said range at both sides of this half height with a sufficiently low spherical aberration. This provides the possibility of scanning a stack of information layers by means of a scanning beam which is compensated once for spherical aberration. A device suitable for scanning a record carrier having a single stack then only needs a single, fixed spherical aberration compensation. Since this compensation can be built into a component which is already present in the device, for example an objective lens of the focusing means, the construction of the device can be simplified considerably.
It is to be noted that the abstract of the Japanese Patent Application no. 60-202 545 describes an information storage system in which the scanning spot of a radiation beam can be varied in height so as to focus on one of the information layers, located at different heights, of a record carrier. However, this publication does not describe the problems which are caused by spherical aberration due to the different heights of the information layers and consequently does not indicate how this aberration should be compensated for.
Since the spherical aberration due to the traversed material thickness of the record carrier is dependent on the refractive index of the material and on the numerical aperture of the radiation beam. the size of the above-mentioned range of sufficiently small spherical aberration will also depend on these parameters. Since the height of the outermost information layers should be within this range for a correct scanning, the maximum thickness of the stack is preferably a function of the refractive index of the material of the stack and of the numerical aperture of the focusing means.
The size of the range of sufficiently small spherical aberration within which information layers can still be scanned with a sufficient quality is determined by the admissible deterioration of the quality of the scanning spot, as determined by the scanning device. The deterioration leads to a less satisfactory detection of electric signals derived from the radiation coming from the record carrier. The deterioration which is maximally admissible for a specific information storage system may be expressed in terms of the Strehl intensity. The Strehl intensity is the normalized maximum intensity of the radiation distribution of the scanning spot. If there are no aberrations, the Strehl intensity is 1, and for large aberrations the Strehl intensity goes towards 0. If the maximally admissible decrease of the Strehl intensity due to spherical aberration is given by r, the size of the range depends on r and the maximum thickness of the stack preferably depends also on r.
The thickness of the stack is preferably smaller than the value 2d defined by the equation       2    ⁢    d    =            34      ⁢              xe2x80x83            ⁢              n        3            ⁢      λ      ⁢              r                            (                              n            2                    -          1                )            ⁢              xe2x80x83            ⁢                        (          NA          )                4            
in which n is the refractive index of the spacer layers, xcex is the vacuum wavelength of the radiation beam and NA is the numerical aperture of the focusing means. If the information layers have such a thickness that the beam is also noticeably influenced by the refractive index of the information layers. the parameter n should be a weighted average of the refractive indices of the spacer layers and the information layers instead of the refractive index of the spacer layers. If the refractive indices of the spacer layers and/or those of the information layers are different, the parameter n should be a weighted average of these different refractive indices.
If the information layers are scanned through a transparent substrate, the spherical aberration caused in this substrate should also be compensated for in the focusing means. Generally, the substrate has small thickness variations within a certain thickness tolerance. If the spherical aberration due to a substrate having a nominal thickness is compensated for, the thickness variations give rise to uncompensated spherical aberration in the radiation beam. This extra spherical aberration takes up a part of the above-mentioned maximally admissible spherical aberration of the information storage system, so that the admissible spherical aberration incurred in the stack is reduced. An information storage system in which scanning through a substrate is realised and in which the extra spherical aberration is taken into account is characterized in that the thickness of the stack is smaller than 2d minus the thickness tolerance of the substrate.
The guide focus is held on a track in the guide plane by a tracking servo. During writing, when there are still no tracks in the recording plane, the write focus is coupled to the guide focus as regards the transversal position, i.e. the position in a direction perpendicular to the optical axis as well as to the tracks. During reading the guide focus is held on the written tracks by means of an active control in the transversal direction. The read focus is then also actively focused on the recording plane to be read.
It is to be noted that Japanese Patent Application 63-298836 describes a method using a guide beam and a write beam. However, each of these beams is focused by a separate objective system so that the coupling between the two beams cannot be realised with sufficient accuracy. Moreover, the latter Patent Application does not describe the writing and reading of record carriers having various recording planes.
If the method according to the invention is further characterized in that the read beam used during reading or erasing is constituted by the guide beam, it can be implemented with a small number of means and the apparatus for performing the method can be simplified.
For writing a recording plane in a record carrier in which the recording planes are constituted by surfaces of separate recording layers which are separated by spacer layers, the method is preferably characterized in that the write focus is held in the recording plane by means of a focus error signal which is obtained from the cooperation of the write beam with the scanned recording plane. The write beam will then remain satisfactorily focused on the recording plane. even if the guide plane and the recording plane are not parallel within a focus depth.
For writing a recording plane in an unlaminated record carrier the method is preferably characterized in that the longitudinal position of the write focus is guided by the longitudinal position of the guide focus, the distance between the two positions being determined by the ordinal number of the recording plane to be written. Use of this method leads to a record carrier in which one or more recording planes are formed.
The Apparatus
A second aspect of the invention relates to an apparatus for performing the method, which apparatus comprises at least one radiation source for supplying a guide beam and at least one write beam, an objective system for focusing the guide beam to a guide focus as well as for focusing the write beam to a write focus, and a first servosystem for longitudinally positioning the guide focus in the guide plane. Such an apparatus is known from the afore-mentioned Japanese Patent Application 62-68207. The drawback of this apparatus is that the read/write beam does not have any independent servosystems for longitudinally and transversely positioning the read/write focus.
Another object of the invention is to provide an apparatus which does not have these drawbacks. This apparatus is characterized in that it comprises a second servosystem for transversely positioning the guide focus in the guide plane, a coupling of the control of the transversal position of the write focus to the second servosystem, a read tracking servosystem and a read focus servosystem for transversely and longitudinally positioning, respectively, a read focus formed by a read beam, said servosystems using a tracking error signal and a focus error signal, respectively, generated by means of the read beam. Spherical aberration is corrected as described below.
The second servosystem holds the guide beam on the tracks in the guide plane. During writing the transversal position of the write focus is coupled to that of the guide focus because there is no tracking information in an unwritten recording plane. During reading the read focus must be held on the tracks in the recording plane by its own tracking servosystem. A coupling of the transversal position of the read focus to that of the guide focus as used during writing cannot be used during reading because the transversal positioning of the write focus with respect to the guide focus during writing cannot be reproduced with sufficient accuracy during reading. For similar reasons the read beam must have its own focus servosystem. To this end an embodiment of the apparatus according to the invention for writing a record carrier in which the recording planes are constituted by surfaces of separate recording layers which are separated by spacer layers is characterized in that the apparatus comprises a third servosystem for longitudinally positioning the write focus in a recording plane, using a focus error signal which is supplied by the write beam. By virtue of the third servosystem the write focus remains in a the recording plane, independent of the parallelism of the recording plane and the guide plane.
A further embodiment of the apparatus according to the invention for wnting a recording plane in an unlaminated record carrier is characterized in that the apparatus comprises a coupling of the control of the longitudinal position of the write focus to the first servosystem. In an unlaminated record carrier a recording plane is not formed until the information is written. Prior to writing, the recording plane is not present so that a write focus cannot be adjusted. For this reason the longitudinal position of the write focus must be coupled to the guide focus during writing.
An embodiment of the apparatus according to the invention may be further characterized in that the third and a fourth servosystem determine the longitudinal and transversal positions, respectively, of the read focus. Reading and writing can then be effected by means of the same radiation beam. In this case the apparatus requires only four servosystems for the guide, write and read beams.
A preferred embodiment of the apparatus according to the invention is characterized in that the first and second servosystems determine the longitudinal and transversal positions, respectively, of the read focus. The same radiation beam can now be used as a guide beam and as a read beam. The apparatus then only requires three servosystems.
To be able to separate the different radiation beams in the apparatus in order to detect them separately, the beams may have a different wavelength, a different state of polarization or a different spatial direction, or a combination thereof.
If the detection systems for the guide beam and read or write beam on the one hand and the radiation sources on the other hand are located at different sides of the record carrier, the advantage is obtained that the power of a radiation beam to be detected is independent of the ordinal number of the scanned recording layer.
When using multiple recording plane record carriers, it is necessary to detect a recording plane having a desired ordinal number. When writing and reading a multilayer record carrier it is not sufficient for the scanning beam, or write/read beam, to be exactly focused on a recording plane, but it is at least as important that the correct, i.e. selected plane is focused. An apparatus providing this possibility is characterized by a recording plane selector which comprises a plane discriminator connected to the output of a focus error detection system of the scanning beam, a counter connected to said discriminator and a comparison circuit for comparing the counter contents with the ordinal number of a recording plane to be scanned.
The plane selection method used in this apparatus differs considerably from and is more reliable than that described in Japanese Patent Application 62-68207, which works with fixed longitudinal distances between the scanning focus and the guide focus.
The presence of the focus servosystem for the read focus is a previously mentioned aspect of the invention. The signal supplied by the focus error detection system comprises information about the presence of a recording plane on or near the scanning focus. When the scanning focus is moved through the recording planes, the plane discriminator can derive a pulse from the above-mentioned signal at any instant when a recording plane passes through the scanning focus. With reference to these pulses and the direction of movement of the scanning focus the counter determines the ordinal number of the recording plane passing the scanning focus. In this way it is possible to focus the scanning beam on any desired layer. The plane selection method according to the invention is applicable to all apparatuses for writing, reading or erasing multilayer optical record carriers which actively focus on the different recording planes in the record carrier.
The second aspect of the invention includes provision for correction of optical aberrations in the scanning beam for the different longitudinal positions of the scanning focus in the record carrier. The thickness of the record carrier traversed by the scanning beam, from the outside surface to the scanning focus, is dependent on the ordinal number of the recording plane to be scanned. This variable thickness introduces a variable quantity of spherical aberration in the scanning beam, which spherical aberration detrimentally influences the shape of the scanning focus. In the case of thickness variations of more than approximately 100 xcexcm the scanning beam must be corrected so as to maintain a satisfactory quality of the scanning focus. To this end an apparatus in accordance with this aspect of the invention is characterized in that the apparatus includes at least one spherical aberration corrector for an adjustable correction of spherical aberration in the scanning beam, the magnitude of the correction being dependent on the refractive index and the thickness of the material of the record carrier in the optical path of the scanning beam bet, en the objective system and the scanned recording layer. A particular embodiment of such an apparatus is characterized in that the spherical aberration corrector having an adjustable correction is a transparent plate comprising a plurality of areas of different thicknesses, each time one of said areas being present in the path of the scanning beam. Plane-parallel plates of, for example, glass or plastics material can easily be used to correct the spherical aberration for any recording plane to be scanned.
It is to be noted that U.S. Pat. No. 3,999,009 also describes an apparatus for scanning a multilayer record carrier, which apparatus has a transparent plate which can be introduced into the scanning beam. This plate is intended to move the scanning focus longitudinally, with the objective system being stationary. In contrast to the plate according to the invention, the known plate does not correct the spherical aberration, but aggravates it. A further difference between the two plates is that the known plate should become thicker to move the scanning focus away from the objective system, and that the plate according to the invention should become thinner.
The spherical aberration corrector may be generally used in an apparatus for scanning multilayer record carriers, not only in an apparatus using a guide beam and a scanning beam, but also in an apparatus without a guide beam.
The invention also relates to a device for scanning information layers of an optical record carrier, which device is provided with a radiation source, focusing means for selectively focusing a radiation beam from the radiation source on separate information layers, and a focus servosystem comprising a focus detection system having a radiation-sensitive surface. Currently unscanned information layers yield a relatively large defocused, parasitic radiation spot on the radiation-sensitive surface. To minimize the crosstalk due to this parasitic spot on the focus error signal. the radiation-sensitive surface should be small. On the other hand, the radiation-sensitive surface should have a certain minimum dimension to enable it to generate a satisfactory focus error signal. According to the invention, the device is therefore characterized in that the radiation-sensitive surface has a largest dimension ranging between 1.5 and 3 times the diameter of the radiation spot formed on the radiation-sensitive surface when the radiation beam is optimally focused on the information layer to be scanned. The above-mentioned 8 xcexcm peak-to-peak distance of the S curve can be realised with such a focus detection system. The maximum dimension of the radiation-sensitive surface is preferably approximately equal to twice the diameter of said radiation spot. It is possible to use a detection system with a relatively large radiation-sensitive surface while meeting the above dimensional requirement by arranging a diaphragm in the radiation beam. restricting the extent of the area of the radiation-sensitive surface on which radiation is incident. The reduction of the crosstalk between the focus error signals makes it possible to reduce the minimum distance between the information layers.
Generally, a device for scanning record carriers is provided with a tracking servosystem for causing the scanning spot of the radiation beam to follow information layer tracks in which the stored information is arranged. Such a servosystem will also be influenced by crosstalk due to currently unscanned information layers situated proximate to the scanning spot. To minimize this influence, the radiation-sensitive surface of the tracking detection system preferably has a dimension ranging between 1 and 3 times the diameter of the radiation spot on the tracking detection system when the radiation beam is optimally focused on one of the information layers. Such a device is particularly suitable for integration in a storage system according to the first and second aspect of the invention.
Some types of the device are provided with dividing means located in the optical path between the radiation source and the focusing means for generating two servobeams and one main beam from the radiation beam, the two servobeams being used for generating a tracking error signal. According to the invention, crosstalk of tracking error signals can be reduced in such a device by choosing the power in the main beam to be smaller than six times, and preferably 4 times, the power in each of the servobeams. The crosstalk appears to be caused by parasitic radiation of the main beam reflected by an information layer which is not to be scanned currently and is incident on the servodetection system. In the device according to the invention the amount of radiation in the servobeams is larger than the amount of parasitic radiation of the main beam at the location of the radiation-sensitive detectors of the tracking detection system. This reduces the influence of the parasitic radiation and thus the crosstalk, thereby allowing a smaller spacer thickness.
The Record Carrier
The method and apparatus according to the invention provide the possibility of using a record carrier which cannot only be written in a well-defined manner but also be read satisfactorily. This record carrier is characterized in that the record carrier has a recording layer of such a thickness that it can be provided with different recording planes which can be scanned separately. Such a thick recording layer can be made at a lower cost than a stack of recording layers and intermediate layers. The recording planes are not formed until the thick recording layer is written.
A preferred embodiment of the record carrier is characterized in that the guide plane has an inscribable layer. The number of recording planes in the recording layer is extended by one by providing the guide plane with a sensitive layer.
A further preferred embodiment of the record carrier is characterized in that the guide plane comprises non-erasable information which has been prerecorded during the production of the record carrier. The non-erasable information enhances the facilities of use of the record carrier and provides, for example, the possibility of distributing standard data or programs. The non-erasable information can be laid down in the guide plane simultaneously with the tracking information by means of a stamping process, as described in, for example GB Patent Application 2,036,410.
The information in the recording planes is preferably coded in accordance with a self-clocking recording code. Since the recording planes do not comprise any synchronizing marks provided during production, the clock for decoding the signal which has been read from the written information in the recording planes, must be generated by the apparatus itself from said signal.
Still another aspect of the invention is related to rendering the information contents of a record carrier as large as possible by maximizing the number of information layers. However, the number of information layers fitting in a stack is limited by the maximum thickness of the stack on the one hand and by the required minimal mutual distance of the information layers on the other hand. The minimum mutual distance is determined by crosstalk of the information layers, i.e. the quality of signals generated from radiation coming from a layer to be scanned is detrimentally influenced by radiation coming from other information layers. The extent of influence is dependent on the type of signal. The minimum distance between information layers for an acceptable crosstalk between the information signals of the different layers is known from European Patent Application no. 0 605 924 which is herein incorporated by reference. Notably. servo-error signal crosstalk gives rise to additional requirements for the minimum distance. In a focus servosystem, with which the device keeps the scanning spot on the information layer to be scanned, the shape of the S-curve, i.e. the response curve of the focus servosystem as a function of the distance between the scanning spot and the information layer, is influenced by a neighbouring information layer. The S-curve generally has a positive and a negative extreme, while the zero crossing in between is the point towards which the servosystem controls the position of the scanning spot. In accordance with the invention, the thickness of each spacer layer in the stack ranges between 1.5 and 4 times the refractive index of the spacer layer multiplied by the peak-to-peak distance of the S-curve of the focus servosystem. A thickness below said range gives rise to a large crosstalk, whereas a thickness above said range gives an-unnecessary decrease of information density of the record carrier. The height is preferably approximately twice the refractive index of the spacer layer multiplied by the peak-to-peak distance. The minimum height of a spacer layer is approximately 18 n xcexcm for a focus servosystem with a peak-to-peak distance of 12 xcexcm. A special focus servosystem designed for scanning multi-layer record carriers has a peak-to-peak distance of 8 xcexcm, so that the minimum thickness of the spacer layer is 12 n xcexcm. Although the features of the second aspect of the invention can be advantageously implemented in an information storage system with the specific correction of the spherical aberration according to the first aspect of the invention, the application is not limited thereto.
The invention further relates to an optical record carrier having a plurality of information layers at different heights in the record carrier, which information layers are separated by spacer layers, which record carrier is suitable to be read by means of a focused radiation beam employing a fixed spherical aberration compensation. According to the invention, the record carrier is characterized in that the distance between the highest and lowest information layer is smaller than a value 2d defined by       2    ⁢    d    =            34      ⁢              xe2x80x83            ⁢              n        3            ⁢      λ      ⁢              r                            (                              n            2                    -          1                )            ⁢              xe2x80x83            ⁢                        (          NA          )                4            
in which n is the refractive index of the spacer layers. xcex is the vacuum wavelength of the focused radiation beam. NA is the numerical aperture of the focuse,i radiation beam and r is 0.05. The numerical aperture of a beam is equal to the sine of the half apex angle of the beam in vacuo. Such a record carrier can be scanned by a relatively cheap scanning device, thereby reducing the cost of the information storage system.
A system in which stringent requirements are imposed on the quality of the scanning spot requires a record carrier in which the height of the stack is smaller than has been indicated in the previous paragraph. According to the invention, such a record carrier is characterized in that the value of r is 0.01.